Periodic interface calibration for high speed communication

ABSTRACT

A high-speed communication interface manages a parallel bus having N bus lines. N+1 communication lines are established. A maintenance operation is performed on one of the N+1 communication lines, while N of the N+1 communication lines is available for data from the N line bus. The communication line on which the maintenance operation is performed, is changed after the operation is complete, so that all of the N+1 communication lines are periodically maintained, without interfering with communications on N of the N+1 communication lines.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present application relates to high-speed communication interfaces,including high-speed parallel bus interfaces for integrated circuits;and more particularly to calibration of such interfaces.

2. Description of Related Art

High-performance data processing applications are driving the demand fordata rates past the GigaHertz range. As processor clock speeds increaseto meet the demand, high-performance parallel bus interface technologyis being developed to meet these needs. In parallel bus interfaces, anumber of serial lines are operated in parallel. So-called SERDES (shortfor serializer-deserializer) technologies are being applied for each ofthe parallel lines. Other high-performance bus interface technologiesare provided by Rambus, Inc., including products provided under thetradenames XDR™ High Performance Memory Interface Technology, Raser™High Performance Interface Technology, and Redwood™ High PerformanceParallel Bus Interface Technology. Background concerning high speedinterfaces is found in U.S. Pat. No. 6,396,329 B1, entitled Method andApparatus for Receiving High Speed Signals with Low Latency; and in U.S.Pat. No. 6,473,439, entitled Method and Apparatus for Fail-SafeResynchronization with Minimum Latency.

One problem which becomes more important as communication speedsincrease is calibration of clocks and sample timing. The optimalsampling point for each bit of data is controlled by many independentvariables, which can be boiled down to a simple relationship betweenclock and data. There is an optimal singular sampling point for all datapatterns at any given moment. Complicating matters are changes to theoptimal sampling point. High-frequency noise, known as jitter, places acloud of uncertainty around this optimal sampling point. Methods tocompensate for jitter have been limited in effectiveness. Thus, systemswith very low jitter are preferred. Low-frequency noise, known as skew,comprises slowly changing offsets in the optimal sampling point, forwhich compensation can be provided, depending on the system's ability totrack of these sources of error.

Several methods have been developed to track and calibrate the sourcesof error that cause skew. One method is known as oversampling.Oversampling requires sampling the data more than once per bit time andcoding the data for guaranteed transitions. These oversamplingapproaches involve clock/data recovery schemes that use clock/datapatterns such as 8b/10b, and the like. Most current SERDES technologiesuse the 8b/10b coding scheme. This approach has the advantage that itrelies on the same number of physical channels as logical channels forthe communication link. However, there is an inherent 25% bandwidthpenalty built-in the 8b/10b coding scheme. Also, the oversamplingrequires increased power consumption.

Another method for tracking and calibrating sources of error of involvesperforming an initial calibration, and then letting the system run openloop. This process requires good circuits to track alltemperature-related drift components. One well-known example of thisapproach is known as the source synchronous technique. A timingreference is sent, typically on an independent physical channel, alongwith the data to compensate for drift between clock and data. Thetracking time constant needs to be as fast as possible, with minimumtime lag. Additionally, a single offset value would be optimal for alloperating conditions on each of the lines in the parallel bus. If goodtracking can be achieved across all drift conditions on all of the linesin the parallel bus, a source synchronous approach is quite compelling.

In another approach, where tracking times are not optimal, each link canbe temporarily disabled and used for a fast periodic calibration. Thistype of periodic calibration requires precise logical synchronizationbetween transmit and receive operations to perform the calibrationefficiently during a calibration window, without jeopardizing real datain the process. Although synchronized periodic operations may bepossible in a master-slave implementation, peer-to-peer periodicoperations may be too prohibitive to be efficiently incorporated.

Selecting an optimal chip-to-chip interconnect strategy relies not onlyon the traditional metrics of latency and effective bandwidth, but alsothe area and power required to do so. System solutions that providesuperior area/bandwidth and power/bandwidth trade-offs, while stillmeeting the bandwidth and latency requirements of system designers, arerequired to continue to scale performance in line with expected trends.

SUMMARY OF THE INVENTION

The present invention provides a communications interface, includingtransmitters and receivers, adapted for periodic calibration, and amethod for maintaining calibration of communication paths across theinterface. The periodic calibration process can operate substantiallycontinuously, as a background process during operation of the interfaceto maintain the communication lines during long intervals of constantuse without reset or other initialization events that allow time fortypical line maintenance operations.

A method according to the present invention manages a high-speedcommunication interface for a parallel bus having N bus lines at thelogical layer. In the physical layer, N+1 communication lines areestablished. A maintenance operation (calibration for example) isperformed on one of the N+1 communication lines, while N of the N+1communication lines is available for data from the N line parallel bus.The communication line on which the maintenance operation is performed,is changed after the operation is complete, so that all of the N+1communication lines are periodically maintained, without interferingwith communications on N of the N+1 communication lines.

Where the maintenance operation is calibration, a calibration signal,such as a pseudorandom bit sequence adapted for calibration of receiverclocks, is transmitted from a source, and received at a destination, onone particular communication line, referred to as communication line(n), of the N+1 communication lines. At the same time, a path ismaintained for communication of data on N communication lines. Aparameter associated with communication line (n) is calibrated. Then,after calibrating the parameter associated with communication line (n),the index (n) is changed and the process is repeated for a nextcommunication line. Accordingly, one of the N+1 communication lines isused for calibration at a time, and is rotated according to a pattern sothat each of the N+1 communication lines is calibrated over time.

Embodiments of the method include entering a reduced power consumptionstate on at least one of receivers and transmitters on the Ncommunication lines, for example when data is not being supplied fromthe N line bus, while continuing to perform the periodic maintenanceoperation on the communication lines. In this manner, powerdown statesare supported without losing maintenance, such as calibration, of thehigh-speed parallel data interface. In one embodiment, the systemincludes a “nap” mode, during which the periodic maintenance procedurecontinues, while other circuitry supporting the communication lines arein a power down state. During the “nap” mode, the maintenance proceduremay operate with a cycle time that is less than, the same as or greaterthan the cycle time during normal operations of the communication lines.The system may also support a mode in which the maintenance proceduresare stopped, and circuitry supporting the maintenance procedures is in apower down state.

The present invention also provides a method for switching thecommunication line subject of maintenance, without interruptingdataflow. The method includes, for example, changing the index (n) toswitch a first particular communication line from being subject ofmaintenance to communicating from a line on the N line bus, and a secondparticular communication line from communicating from the line on the Nline bus to being subject of maintenance, routing the first and secondparticular communication lines together from the line in the N line busduring a settling interval, and then, after the settling interval,performing maintenance on the second particular communication line.

The index (n) is changed in embodiments of the present inventionaccording to a continuous periodic function, so that each of the N+1communication lines is maintained at least once during a period of thecontinuous periodic function. Where the set of N+1 communication linesincludes communication lines logically identified as paths 0 to N, onepattern comprises a repeating pattern beginning with the index (n) equalto zero, and increasing to (n) equal to N, and then decreasing to (n)equal to zero.

The present invention is also embodied by signal interfaces supportingthe source and destination ends of the communication lines. Thus, anembodiment of the invention includes a set of signal lines having N+1signal lines and N+1 receivers coupled to respective signal lines in aset of signal lines, which together establish a set of N+1 signal paths.The set of N+1 signal paths is adapted to serve an N line bus. A linemaintenance circuit, such as the calibration circuit, is included in theinterface. A switch placed in the N+1 signal paths, such as between theN+1 receivers and the N line bus, and control logic for the switch,operate to selectively route N signal paths in the set to the N line busand one signal path, signal path (n), in the set to the line maintenancecircuit. The index (n) is changed as discussed above to maintain thesignal paths in the set without interfering with dataflow.

According to embodiments of the invention, the line maintenance circuitcomprises a calibration circuit. For example, a calibration circuit isused to set adjustable clock generators that are used to supply receiverclocks for each of the N+1 receivers.

In yet other embodiments, logic is included to power down the N+1receivers while continuing to maintain signal paths in a set of signalpaths according to the pattern.

Other embodiments of the invention are implemented on the source side ofthe communication line. In such embodiments, an N line bus feeds a setof signal lines having N+1 signal lines. N+1 transmitters are coupled tothe set of signal lines establishing a set of N+1 signal paths. A linemaintenance circuit is included. A switch is coupled to the N+1 signalpaths. Control logic for the switch selectively routes N signal paths inthe set from the N line bus to N signal lines in the set of signallines, and routes one signal path, signal path (n), in the set from theline maintenance circuit to the signal line (n). The index (n) ischanged so that the line maintenance circuit is successively coupled toeach of the N+1 signal lines according to the pattern, such as describedabove. Likewise, on the source side of the communication line, the N+1transmitters can be powered down without interfering with the linemaintenance process.

Further embodiments of the invention comprise the combination of thesource side, destination side and communication media to provide acomplete high-speed, parallel communication system.

Embodiments of the present invention support communication betweenintegrated circuits at data rates greater than 100 MHz, and in someembodiments greater than 1 GHz, and more.

The example of line maintenance mentioned above involves transmission ofcalibration signals used for example for calibration of receiver clocks.Calibration can be applied to other parameters of the communicationline, such as signaling levels, optimal placement of sampling times forsymbol capture, and impedance of the termination element, and receiverthresholds. For communication lines that use adaptive equalization orfilters, the maintenance can include adjustment of equalization orfilter coefficients. The line management process can be applied to linemaintenance applications which may or may not involve transmission ofcalibration signals.

Other aspects and advantages of the present invention can be seen onreview of the drawings, the detailed description and the claims, whichfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a system employing periodiccalibration.

FIG. 2 is a more detailed diagram of physical layer signal paths in asystem using continuous periodic calibration.

FIG. 3 shows flowcharts for continuous periodic calibration on thetransmit side and on the receive side.

FIG. 4 illustrates circuitry for calibration of a receive clock in acircuit such as shown in FIG. 2.

FIG. 5 is a simplified block diagram of a system employing periodiccalibration, in combination with a source synchronous clock.

DETAILED DESCRIPTION

A detailed description of embodiments of the present invention isprovided with reference to FIGS. 1-5.

FIG. 1 is a simplified block diagram of the communication systemapplying continuous periodic calibration according to the presentinvention. The system includes a first integrated circuit 10 and asecond integrated circuit 11. The first integrated circuit 10 includes alogical layer parallel bus 20 including N lines, a calibration signalsource 21, and calibration logic 22. A switch 23 couples the parallelbus 20 and the calibration signal source 21 with a set of transmitters12-16, including one for each of N+1 physical layer communication lines.The set of transmitters 12-16 drives communication signals acrosscommunication media. In this example, the set of transmitters 12-16drive data on signal lines coupled to input/output ports 32-36 (such asIO pins on the integrated circuit), which are coupled to respectivetransmission lines, including line 0 through line N in a set of N+1transmission lines.

The second integrated circuit 11 includes complementary components.Input/output ports 42-46 are coupled by signal lines to respectivereceivers 52-56 in a set of N+1 receivers on the second integratedcircuit 11. The receivers 52-56 are coupled to switch 57. Switch 57routes the outputs of N receivers from the set to an N-line parallel bus58, while routing the output of one of the receivers from the set to acalibration circuit 59. Calibration logic 60 on the second integratedcircuit 11 controls the switch 57 and calibration circuit 59 to managethe continuous periodic calibration of the set of communication lines.

The logic 22 in the first integrated circuit 10 and the logic 60 in thesecond integrated circuit 11 support a nap state, in which thetransmitters and receivers are placed in a power down mode when notneeded, while the calibration cycle continues. This nap state maintainsreadiness of the high-speed parallel interface for fast transition frompower conserving conditions to awake operations in the transmitting andreceiving systems. In some embodiments, another low power state isincluded in which the calibration process is also stopped.

In FIG. 1, the communication links are shown operating in one direction.The invention is also extended to bidirectional communication links,where the receivers and transmitters, and other supporting logic, arefound on both the first and second integrated circuits.

The continuous periodic calibration process uses an extra link toprovide a mechanism to time multiplex the calibrating operation acrossan interface. In a parallel interface with eight logical links (N=8),nine physical links would be used with one assigned in a rotatingpattern to be the calibration link. The calibration link spends as muchtime calibrating as necessary, without affecting worst-case latency ofthe system. The rate of rotation among the set of communication linkscan be adapted to suit the needs of the particular implementation. Forexample, in systems applying spread spectrum clocking, in which theclock rate is varied over a relatively slow interval, the rate ofrotation among the communication links should be high enough that thechanges in clock rate due to spread spectrum processing are notimpacted. In other examples, the rate of rotation should be fast enoughto accommodate known sources of skew of the parameter being calibrated,such as temperature drift coefficients.

The continuous periodic calibration process can be extended to gluemultiple parallel interfaces together, for example in a daisy chainconfiguration. The process yields a worst-case reduction in overalleffective bandwidth to 1/N+1, where N is the number of logical links inthe system. The rotation of the calibration link is done entirely in thephysical layer in preferred embodiments, providing a seamless N linklogical layer to the host system.

One difficulty can arise in the handoff between the rotating calibrationlink and a regular data link, especially in cases without any backchannel communication supporting the rotation of the calibrationoperation. The handoff must be executed without loss of data in thetransition, and requires some synchronization between the transmit sideand the receive side.

FIG. 2 illustrates one particular implementation of the physical layerin a high-speed parallel communication interface according to thepresent invention. With an input bus supplying eight bits of data (N=8),eight transmit data sources TDATA[0] to TDATA[7] (100-107) provide eightinputs from the logical layer. A transmit calibration signal sourceTXCAL (108) provides a ninth input. A switch includes nine physicallayer, three-input multiplexers 110-118 having outputs coupled torespective transmitters TX_IO[0] to TX_IO[8] (120-128), which drive dataon respective communication media 130-138. The inputs to themultiplexers 110-118 can be characterized with respect to the index (n),where (n) is an integer from 0 to N corresponding to the N+1communication media 130-138. Each multiplexer (n) has as input theoutput of the transmit calibration source TXCAL 108, and input bus linesTDATA[n−1] and TDATA[n], except on the boundaries where multiplexer (0)110 receives the input bus line TDATA[0] only, and multiplexer (N=8 to)118 receives the input bus line TDATA[7] only. Extra inputs on themultiplexers 110 and 118 on the boundary may be used to supportdaisy-chaining multiple buses.

On the receive side, N+1 receivers RX_IO[0] to RX_IO[8] (140-148) arecoupled to respective communication media 130-138. The outputs of thereceivers 140-148 are each coupled to respective buffers 150-158 whichdrive the outputs to a receiver calibration circuit RXCAL 171. Inaddition, a switch includes eight physical layer, two-input multiplexers160-167 coupled to the outputs of the receivers 140-148. The inputs tothe multiplexers 160-167, can be characterized with respect to the index(n), where (n) ranges from 0 to N−1. Thus, the input two multiplexer(n)in the set of multiplexers 160-167 on the receive side include theoutputs of receivers RX_IO[n] and RX_IO[n+1]. The outputs of themultiplexers 160-167 are coupled to the logical layer N line bus 170,providing a receive data RXDATA[7:0] (note that RXDATA[7:0] can bethought of as performing the reciprocal function of the TDATA[0]100-TDATA[7] 107).

Control logic associated with the multiplexers on both sides manages thehandoff between changing operation of a first particular communicationlink for calibration to communicating data, and changing operation of asecond particular communication link from communicating data tooperation for calibration. In one example, the link being calibrated isrotating among the set of N+1 communication lines according to acontinuous periodic pattern, as can be understood with reference to anexample, as follows. Assume that the communication link corresponding toindex (n=1), comprising transmitter TX_IO[1] 121, communication medium131, and receiver RX_IO[1], is currently assigned to the calibrationtask. In this state, the transmit data lines TDATA[7:0] from the inputbus map to transmitters TX_IO[8:2,0]. Likewise, receivers RX_IO[8:2,0]map to receive data lines RXDATA[7:0]. The transmitter TX_IO[1] istransmitting calibration data, and the receiver RX_IO[1] is coupled tothe receive calibration circuit RXCAL 171 via buffer 151. Aftercalibration is complete on the receiver RX_IO[1], such as by adjustingthe clock to the calibrated sampling point, the system is ready tochange the link on which calibration is executed. An operation to switchthe communication link assigned to the calibration task from index (n=1)to index (n=2) is executed as follows.

A) TDATA[1] is mapped to both transmitters TX_IO[1] and TX_IO[2].

B) The receiver RX_IO[1] is mapped to RDATA[1] at multiplexer 161, sothat both RX_IO[2] and RX_IO[1] are transmitting the same data to themultiplexer 161.

C) RX_IO[2] is coupled to the receive calibration circuit RXCAL 171, andRX_IO[1] is selected by multiplexer 161 to apply data to RDATA[1].

D) TX_IO[2] is switched to the calibration signal source TXCAL 108.

E) RX_IO[2] starts supplying calibration data to the receive calibrationcircuit RXCAL 171.

At the completion of these steps, the input bus TDATA[7:0] is mapped viathe transmitters TX_IO[8:3,1:0] across the media 138-133, 131 and 130 tothe receivers RX_IO[8:3,1:0], to output bus RXDATA[7:0]. This operationoccurs without interruption in the logical layer communications. Aftercompletion of calibration on the signal path including RX_IO[2], theprocess waits for TX_IO[2] to begin transmitting data once again. Then,the link being calibrated is changed to the next communication line.

Logic on the receive and transmit sides coordinates the changing of thecalibration link. One simple approach would be to provide back channelcommunication such as operation codes in the logical layer thatcoordinate synchronizing rotation of the calibration link. However, thisadditional complexity at the logical layer may not be necessary in someembodiments. Another approach would be to use internal counters on bothsides of the link synchronized during an initialization. With sufficienttiming padding around the transition points, accuracy of thesynchronization requirements could be reduced allowing each side tooperate essentially open loop, with the possible exception of aninitialization routine which establishes a starting point.

FIG. 3 illustrates one process for coordinating the changing of thecalibration link. On the left side of FIG. 3, a routine for the transmitside is shown. On the right side of FIG. 3, a routine for the receiveside is shown. In the flowcharts, each physical communication link PHYis given the index “i”. For the communication link used for calibrationthe index i=n.

Calibration starts on the transmit side at block 300. At the start,calibration data is transmitted on the physical link PHY[n], and logicaldata is transmitted on the physical links PHY[i] for i<n, and PHY[i+1]for i>n (block 301). This is a representative mapping of the logicaldata to physical links for changing the calibration link in a patternwhere the calibration link changes from link 0 through link N in anincreasing manner. The mapping will be adapted according to the patternused for changing the calibration link. The transmit side waits a timeinterval (ΔT) represented by line 302, which is long enough to allow thereceive side to complete calibration. After waiting a time interval, theprocess switches the calibration path (block 303). According to thisprocess, logical data for input line (n) is transmitted on PHY[n] inparallel with the transmission on PHY[n+1], for a time corresponding toa settling interval at the receiver (block 304). Then, logical data forinput line (n) is transmitted only on PHY[n] (block 305). At this point,the process is ready to change the calibration link, and the index (n)is changed according to a continuous periodic pattern (block 306). Thenthe process loops back to block 301, and repeats.

On the receive side, calibration starts at block 310. To begin theprocess, data for calibration is received on the physical link PHY[n](block 311). The received calibration data is processed, and the logicaldata signals received on the other links are routed to the receiver bus(block 312). Line maintenance or calibration is executed for PHY[n], tofor example update a calibration parameter like clock phase (block 313).The receiver then waits a time interval (ΔT) represented by arrow 314,to provide a margin for synchronization with the transmit side of thehigh-speed parallel bus. After the time interval, the receiver switchescalibration path (block 315). The process to switch the calibration pathincludes receiving logical data for bus line (n) on both PHY[n] andPHY[n+1] (block 316). After a settling interval, PHY[n+1] is switched tothe receive calibration circuit RXCAL 171, while PHY[n] is coupled tothe receiver bus (block 317). Thus, the next communication link PHY[n+1]is ready to receive calibration data. The value of the index (n) ischanged according to the pattern (block 318), then the process loopsback to block 311, and repeats.

According to one embodiment of the present invention, the pattern forthe continuous periodic rotation, referring to FIG. 2, would be asfollows:

n=0, 1, 2, 3, 4, 5, 6, 7, 8, 7, 6, 5, 4, 3, 2, 1, 0, 1 . . .

This pattern simplifies the switching between links, as the handoffoccurs between adjacent links in each step. However, the time betweenupdates is not the same for all links. This difference in time betweenupdates may not be a problem for a small number of links. However,larger error terms may be encountered for the communication systems inwhich many blocks are daisy chained together.

The worst-case update rate is described in equation 1, where N is thenumber of actual data links, and T_(cal) is the total time for a link tocalibrate and handoff to the next link.T _(update)=2*N*T _(cal)  Eq. 1)

Assuming a link takes about 1000 cycles to calibrate a clock at 400 MHz,and adding some time for synchronization, T_(cal) can be approximated asfive microseconds. This means the update frequency for an eight linksystem would be around 12.5 kHz. A reduction of calibration time byfactor of 10 could potentially increase update frequency to about 150kHz, if such a scheme could maintain synchronization and have theaccuracy necessary for a given application. Other items such as spreadspectrum clocking may affect the desired update rate.

By way of example, FIG. 4 is a simplified diagram of a system suitablefor use with the continuous periodic calibration technique of thepresent invention, where the calibration is applied to adjusting clockphase to select optimal sampling point for a physical channel. In thisexample, only the receive side is illustrated. However, both transmitand receive sides receive a reference clock, in this example a 400 MHzclock on line 400. The reference clock is applied to a phase-locked loopcircuit 401 which multiplies a clock by eight in this example to producea receive clock at a frequency of 3.2 GHz. One copy of the receive clockis applied to each of the receivers, such as receiver 141. The receiver141 includes a clock phase adjustment circuit 402 which applies a clockat the calibrated sampling point to receive sense amplifier 403. Inputfrom the physical channel at 6.4 gigabits per second, where samplingoccurs on each transition of clock is received through a buffer 404 intothe sense amplifier 403. The output of the sense amplifier 403 isapplied to the calibration circuit, which comprises the mixer 405, and asource 406 of a pseudorandom bit sequence used for calibration. Thereceived pseudorandom bit sequence RX_PRBS(n) on the physical layer isapplied to the mixer 405. The mixer 405 produces an adjustment parameteron line 407, which is applied to the clock phase adjuster 402. When theoutput of the sense amplifier 403 is used for data, it is applied to aserial-to-parallel converter 408, clocked at the reference clock rate,e.g. 400 MHz, to apply a parallel output at the reference clock rate. Inthis example, translating 6.4 gigabits per second to a 400 MHz clockwould involve an 8-bit serial-to-parallel converter 408 for each of thereceive signal paths.

Although much of the discussion has been addressed to calibration fromthe perspective of timing calibration, the invention can be applied toother functions requiring periodic adjustment of the physical channelfor data transmission, such as current calibration, resistorcalibration, adaptive equalization, and other types of line maintenanceand tuning.

In some embodiments, a hybrid method of sending a source synchronousclock, along with continuous periodic calibration as described above canbe used at the expense of an additional physical layer link to carry theclock. In FIG. 5, the system of FIG. 1 is shown using the same referencenumerals, with an additional physical layer link 500 from a source clock501 on the first integrated circuit 10 to a clock circuit 502 on thesecond integrated circuit used for carrying a source synchronous clock,which may be desirable in some environments where source synchronousclocking provides superior performance, and can be used in combinationwith the continuous periodic calibration process described herein. Theadditional physical layer link 500 carrying the clock may or may not beincluded in the continuous periodic calibration routine, as suits aparticular implementation.

According to the present invention, continuous periodic calibrationprovides a solution for tracking slowly changing drift terms for idealsampling of clock relative to data. It seamlessly calculates the bestsampling point of data continuously, with the overhead of one extra IO,without knowledge of the logical layer.

The technique is particular suited to high-speed chip to chipcommunications.

In summary, the present invention provides methods and apparatus forproviding continuous calibration of properties associated with aparallel interface that includes N links. The calibrated property mayinclude for example signaling levels, optimal placement of samplingtimes for symbol capture, and impedance of the termination element orequalization coefficients associated with one of the N links. In oneembodiment, continuous calibration of an optimal timing point forsampling by receiver circuit is provided using an additional link (N+1)to time multiplex a calibration sequence among the N links. Thecalibration is rotated or switched among the N+1 links, while normalcommunication is executed on the other N links.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims. What is claimed is:

1. A signal interface, comprising: a set of signal lines having N+1signal lines, where N is an integer; N+1 receivers coupled to respectivesignal lines in the set of signal lines establishing a set of N+1 signalpaths with the set of signal lines; an N line bus; a line maintenancecircuit; and a switch in the N+1 signal paths, and control logic for theswitch, which selectively routes N signal paths in the set to the N linebus and signal path (n) in the set to the line maintenance circuit,where (n) is changed according to a pattern to selectively maintainsignal paths in the set of N+1 signal paths while enabling data flow onN signal paths in the set to the N line bus.
 2. The signal interface ofclaim 1, wherein the pattern comprises a periodic pattern.
 3. The signalinterface of claim 1, wherein the set of N+1 signal paths includessignal paths logically identified as paths 0 to N, and the patterncomprises a repeating pattern beginning with (n) equal to 0 andincreasing to (n) equal to N, and then decreasing to (n) equal to
 0. 4.The signal interface of claim 1, wherein the receivers are responsive torespective receive clock signals produced by adjustable clockgenerators, and said line maintenance circuits set the adjustable clockgenerators.
 5. The signal interface of claim 1, wherein the receiversare responsive to respective receive clock signals produced byadjustable clock generators, and said line maintenance circuit sets theadjustable clock generators in response to a calibration data pattern onthe signal path coupled to the line maintenance circuit.
 6. The signalinterface of claim 1, wherein the control logic controls the switch fora change switching a first particular signal path from routing to theline maintenance circuit to routing to a line in the N line bus, and asecond particular signal path from routing to the line in the N line busto the line maintenance circuit so that during a settling interval, thefirst and second particular signal paths are routed together to the linein the N line bus, and then after the settling interval the secondparticular signal path is coupled to the line maintenance circuit. 7.The signal interface of claim 1, wherein the control logic includeslogic for coordinating the pattern with a source of data for the N linebus.
 8. The signal interface of claim 1, wherein said N+1 receivers,said N line bus, said line maintenance circuit; and said switch comprisecomponents of a single integrated circuit.
 9. The signal interface ofclaim 1, including logic to power down the N+1 receivers whilecontinuing to selectively maintain signal paths in the set of signalpaths.
 10. The signal interface of claim 1, wherein the N+1 receiversare adapted to receive data with a data rate higher than 100 MegaHertz.11. The signal interface of claim 1, wherein said N+1 receivers, said Nline bus, said line maintenance circuit; and said switch comprisecomponents of a single integrated circuit, and the N+1 receivers areadapted to receive data with a data rate higher than 100 MegaHertz froma source external to the integrated circuit.
 12. The signal interface ofclaim 1, further including an additional signal line adapted to receivea source synchronous clock.
 13. A signal interface, comprising: an Nline bus; a set of signal lines having N+1 signal lines, where N is aninteger; N+1 transmitters coupled to respective signal lines in the setof signal lines establishing a set of N+1 signal paths with the set ofsignal lines; a line maintenance circuit; and a switch in the N+1 signalpaths, and control logic for the switch, which selectively routes Nsignal paths in the set from the N line bus to N signal lines in the setof signal lines, and routes signal path (n) in the set from the linemaintenance circuit to signal line (n) in the set of signal lines, where(n) is changed according to a pattern to selectively perform maintenanceon signal paths in the set of N+1 signal paths while enabling data flowon N signal paths in the set from the N line bus.
 14. The signalinterface of claim 13, wherein the pattern comprises a periodic pattern.15. The signal interface of claim 13, wherein the set of N+1 signalpaths includes signal paths logically identified as paths 0 to N, andthe pattern comprises a repeating pattern beginning with (n) equal to 0and increasing to (n) equal to N, and then decreasing to (n) equal to 0.16. The signal interface of claim 13, wherein the line maintenancecircuit comprises a calibration signal source that produces a signalpattern adapted for calibration of receive clock signals.
 17. The signalinterface of claim 13, wherein the line maintenance circuit comprises acalibration signal source that produces a pseudo random signal patternadapted for calibration of receive clock signals.
 18. The signalinterface of claim 13, wherein the control logic controls the switch fora change switching a first particular signal path from routing from theline maintenance circuit to routing from a line in the N line bus, and asecond particular signal path from routing from the line in the N linebus to routing from the line maintenance circuit so that during asettling interval, the first and second particular signal paths arerouted together from the line in the N line bus, and then after thesettling interval the second particular signal path is routed from theline maintenance circuit.
 19. The signal interface of claim 13, whereinthe control logic includes logic for coordinating the pattern with adestination of data for the N line bus.
 20. The signal interface ofclaim 13, wherein said N+1 transmitters, said N line bus, said linemaintenance circuit; and said switch comprise components of a singleintegrated circuit.
 21. The signal interface of claim 13, includinglogic to power down the N+1 transmitters while continuing to selectivelyperform maintenance on signal paths in the set of N+1 signal paths. 22.The signal interface of claim 13, wherein the N+1 transmitters areadapted to transmit data with a data rate higher than 100 MegaHertz. 23.The signal interface of claim 13, wherein said N+1 transmitters, said Nline bus, said line maintenance circuit; and said switch comprisecomponents of a single integrated circuit, and the N+1 transmitters areadapted to transmit data with a data rate higher than 100 MegaHertz to adestination external to the integrated circuit.
 24. The signal interfaceof claim 13, further including an additional signal line adapted totransmit a source synchronous clock.
 25. A communication system forinter-chip signals, comprising: a first integrated circuit, a secondintegrated circuit, and a set of N+1 communications lines between thefirst and second integrated circuits; the first integrated circuitcomprising a first N line bus, where N is an integer; a set of signallines having N+1 signal lines coupled to respective communications linesin the set of N+1 communications lines; N+1 transmitters coupled torespective signal lines in the set of signal lines establishing a set ofN+1 transmitter signal paths with the set of signal lines; a calibrationsignal source; and a switch in the N+1 transmitter signal paths, andfirst control logic for the switch, which selectively routes Ntransmitter signal paths in the set from the N line bus to N transmittersignal lines in the set of signal lines, transmitter signal path (n) inthe set from the calibration signal source to one transmitter signalline in the set of transmitter signal lines, where (n) is changedaccording to a pattern to selectively supply calibration signals oncommunication lines in the set of N+1 communication lines while enablingdata flow on N communication lines in the set from the N line bus; andthe second integrated circuit comprising a set of signal lines havingN+1 signal lines coupled to respective communications lines in the setof N+1 communications lines; N+1 receivers coupled to respective signallines in the set of signal lines establishing a set of N+1 receiversignal paths with the set of signal lines; a second N line bus; acalibration circuit; and a switch in the N+1 receiver signal paths, andsecond control logic for the switch, which selectively routes N receiversignal paths in the set to the second N line bus and receiver signalpath (n) in the set to the calibration circuit, where (n) is changedaccording to the pattern to selectively calibrate receiver signal pathsin the set of N+1 receiver signal paths while enabling data flow on Nreceiver signal paths in the set to the second N line bus.
 26. Thecommunication system of claim 25, wherein the pattern comprises aperiodic pattern.
 27. The communication system of claim 25, wherein theset of N+1 receiver signal paths includes receiver signal pathslogically identified as paths 0 to N, and the pattern comprises arepeating pattern beginning with (n) equal to 0 and increasing to (n)equal to N, and then decreasing to (n) equal to
 0. 28. The communicationsystem of claim 25, wherein the calibration signal source produces asignal pattern adapted for calibration of receive clock signals.
 29. Thecommunication system of claim 25, wherein the calibration signal sourceproduces a pseudo random signal pattern adapted for calibration ofreceive clock signals.
 30. The communication system of claim 25, whereinthe first control logic controls the switch for a change switching afirst particular transmitter signal path from routing from thecalibration signal source to routing from a line in the N line bus, anda second particular transmitter signal path from routing from the linein the N line bus to routing from the calibration signal source so thatduring a settling interval, the first and second particular transmittersignal paths are routed together from the line in the N line bus, andthen after the settling interval the second particular signal path isrouted from the calibration signal source.
 31. The communication systemof claim 25, wherein the second control logic controls the switch for achange switching a first particular receiver signal path from routing tothe calibration circuit to routing to a line in the N line bus, and asecond particular receiver signal path from routing to the line in the Nline bus to the calibration circuit so that during a settling interval,the first and second particular receiver signal paths are routedtogether to the line in the N line bus, and then after the settlinginterval the second particular receiver signal path is coupled to thecalibration circuit.
 32. The communication system of claim 25, whereinthe first control logic and second control logic include logic forcoordinating the pattern.
 33. The communication system of claim 25,including logic to power down the N+1 transmitters while continuing toselectively supply calibration signals on transmitter signal paths inthe set of N+1 transmitter signal paths.
 34. The communication system ofclaim 25, including logic to power down the N+1 receivers whilecontinuing to selectively calibrate receiver signal paths in the set ofN+1 receiver signal paths.
 35. The communication system of claim 25,wherein the N+1 transmitters and the N+1 receivers are adapted tocommunicate via the set of communications lines with a data rate higherthan 100 MegaHertz.
 36. The communication system of claim 25, furtherincluding an additional communication line adapted for a sourcesynchronous clock.
 37. A method for managing a high speed communicationinterface for a parallel bus having N bus lines, where N is an integer,comprising: establishing N+1 communication lines; performing amaintenance operation on communication line (n) of the N+1communications lines and enabling paths from the N bus lines on N of theN+1 communications lines; after performing the maintenance operation oncommunication line (n) of the N+1 communications lines, changing (n) andperforming a maintenance operation a next communication line of the N+1communication lines.
 38. The method of claim 37, wherein performing themaintenance operation includes: transmitting a calibration signal oncommunication line (n) from a calibration signal source; receiving thecalibration signal on communication line (n) of the N+1 communicationslines; and calibrating a parameter associated with communication line(n) on the N+1 communications lines in response to the calibrationsignal.
 39. The method of claim 37, including transmitting data from theN bus lines while performing the maintenance operation on communicationline (n).
 40. The method of claim 37, including entering a reduced powerconsumption state on at least one of receivers and transmitters on the Nof the communication lines, while performing the maintenance operationon communication line (n).
 41. The method of claim 37, for a changing(n) to switch a first particular communication line from subject of themaintenance operation to communicating from a line on the N line bus,and a second particular communication line from communicating from theline on the N line bus to subject of the maintenance operation, routingthe first and second particular communication lines together from theline in the N line bus during a settling interval, and then after thesettling interval performing the maintenance operation on the secondparticular communication line.
 42. The method of claim 37, includingchanging (n) according to a continuous periodic pattern.
 43. The methodof claim 37, wherein the set of N+1 communication lines includescommunication lines logically identified as paths 0 to N, and includingchanging (n) according to a repeating pattern beginning with (n) equalto 0 and increasing to (n) equal to N, and then decreasing to (n) equalto
 0. 44. The method of claim 37, wherein performing the maintenanceoperation includes sending a calibration signal from a source oncommunication line (n), the calibration signal comprising a signalpattern adapted for calibration of receive clock signals.
 45. The methodof claim 37, wherein performing the maintenance operation includessending a calibration signal from a source on communication line (n),the calibration signal comprising a pseudo random signal pattern adaptedfor calibration of receive clock signals.
 46. The method of claim 37,further including providing a source synchronous clock.
 47. A signalinterface, comprising: a set of signal lines; a set of receivers coupledto respective signal lines in the set of signal lines; a bus comprisinga set of bus lines; a line maintenance circuit; and a switch coupled tothe set of receivers, to the bus and to the line maintenance circuit,and control logic for the switch, which selectively routes signals inparallel from receivers in the set of receivers to bus lines in the setof bus lines and to the line maintenance circuit, where the receiver inthe set of receivers routed to the line maintenance circuit is changedaccording to a pattern to selectively maintain signal paths over saidset of signal lines.
 48. A transmission circuit on an integratedcircuit, comprising: a line maintenance circuit to output a linemaintenance signal; a set of transmitters coupled to receive a first setof signals and the line maintenance signal, and to output a second setof signals, wherein the second set of signals includes the first set ofsignals and the maintenance signal; and a switch coupled to the set oftransmitters and a control logic for the switch, to selectively routethe first set of signals and the line maintenance signal in parallel tothe set of transmitters, where the transmitter in the set oftransmitters routed to by the line maintenance circuit is changedaccording to a pattern to selectively maintain the second set of signalsfrom the set of transmitters and to permit the maintenance signal to beused as a calibration signal.
 49. A receiver circuit on an integratedcircuit, comprising: means for receiving a first set of signals and aline maintenance signal, and to output a second set of signals; meansfor calibrating the means for receiving, the means for calibratingcoupled to receive the line maintenance signal; means for routing thefirst set of signals and the line maintenance signal in parallel fromthe means for receiving, wherein the routing changes according to apattern to selectively maintain the second set of signals and to permitthe maintenance signal to be used as a maintenance signal formaintaining different portions of the means for receiving.